Well, since you can't be sure what a package will do, the user/admin
should at least have the possibility to examine the contents of a
package, manually or with virus-scanners or whatever without actually
running any script or some unpack-in-place routine provided by the
package itself.

So far, some Installers fail this miserably:

NullSoft Installer
http://nsis.sourceforge.net/
The packager has complete control over any switches given to the
package, and most choose not to provide an "unpack only" switch.
There is no official way to unpack these packages, 7zip and the
likes fail.

Loki Installer
http://www.lokigames.com/development/setup.php3
It seems you can give the parameter --noexec, which will probably
not execute any scripts. Changing this in the prepended script would
probably lead to a wrong md5-sum; but a specially prepared loki-setup
would produce packets which could ignore that. So chances are slim
somebody could change an already existing package, but the packager
himself can do as he sees fit. And then, unpacking such a package
by hand is not very feasible.

For some others are third-party tools available, like
http://innounp.sourceforge.net/ which you can use to examine packages by
hand or plug in into your antivirus.

I consider it mandatory that packages allow being unpacked in place, by
a tool that is not part of the package itself. Its the least thing you
can do.